jdk-24/hotspot/src/share/vm/runtime/basicLock.cpp

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#include "precompiled.hpp"
#include "runtime/basicLock.hpp"
#include "runtime/synchronizer.hpp"

void BasicLock::print_on(outputStream* st) const {
  st->print("monitor");
}

void BasicLock::move_to(oop obj, BasicLock* dest) {
  // Check to see if we need to inflate the lock. This is only needed
  // if an object is locked using "this" lightweight monitor. In that
  // case, the displaced_header() is unlocked, because the
  // displaced_header() contains the header for the originally unlocked
  // object. However the object could have already been inflated. But it
  // does not matter, the inflation will just a no-op. For other cases,
  // the displaced header will be either 0x0 or 0x3, which are location
  // independent, therefore the BasicLock is free to move.
  //
  // During OSR we may need to relocate a BasicLock (which contains a
  // displaced word) from a location in an interpreter frame to a
  // new location in a compiled frame.  "this" refers to the source
  // basiclock in the interpreter frame.  "dest" refers to the destination
  // basiclock in the new compiled frame.  We *always* inflate in move_to().
  // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
  // cause performance problems in code that makes heavy use of a small # of
  // uncontended locks.   (We'd inflate during OSR, and then sync performance
  // would subsequently plummet because the thread would be forced thru the slow-path).
  // This problem has been made largely moot on IA32 by inlining the inflated fast-path
  // operations in Fast_Lock and Fast_Unlock in i486.ad.
  //
  // Note that there is a way to safely swing the object's markword from
  // one stack location to another.  This avoids inflation.  Obviously,
  // we need to ensure that both locations refer to the current thread's stack.
  // There are some subtle concurrency issues, however, and since the benefit is
  // is small (given the support for inflated fast-path locking in the fast_lock, etc)
  // we'll leave that optimization for another time.

  if (displaced_header()->is_neutral()) {
    ObjectSynchronizer::inflate_helper(obj);
    // WARNING: We can not put check here, because the inflation
    // will not update the displaced header. Once BasicLock is inflated,
    // no one should ever look at its content.
  } else {
    // Typically the displaced header will be 0 (recursive stack lock) or
    // unused_mark.  Naively we'd like to assert that the displaced mark
    // value is either 0, neutral, or 3.  But with the advent of the
    // store-before-CAS avoidance in fast_lock/compiler_lock_object
    // we can find any flavor mark in the displaced mark.
  }
// [RGV] The next line appears to do nothing!
  intptr_t dh = (intptr_t) displaced_header();
  dest->set_displaced_header(displaced_header());
}